The determination of the power distribution in large nuclear reactor cores is necessary in order to insure that the core is operated under optimum conditions and within safety limits on the linear rate of heat production in the nuclear fuel rods. Since the beginning of nuclear power, the trend has been to place more and more emphasis on detailed monitoring of the power distribution of the nuclear reactor core, both from the viewpoint of adhering to federal regulations and also from the viewpoint of plant operating efficiency.
The determination of the core power distribution within present large light water reactors is subject to uncertainties which arise from two major areas.
1. The measurements themselves, that is, the detector signals from the many detectors that are located throughout the nuclear reactor core.
2. The analytic conversion of these detector signals into a meaningful parameter, that is, the power distribution within the nuclear reactor core.
The detectors presently used in light water reactors are based upon neutron flux measurement, and these neutron flux measurements are then treated analytically to produce values for the power distribution within the nuclear core.
The measurement errors (item 1) include uncertainty in correction for burnup of detector absorber, manufacturing tolerances, interference from gamma radiation, precise location of the detector within its guide tube, presence of steam voids, coolant temperature, soluble boron content of the coolant, and the exact composition of the fuel bundle, proximity of control rods, and exposure. The analytic conversion of the in-core detector signals (item 2) is a complicated procedure which involves a number of basic assumptions as well as pre-calculated constants from analysis of the characteristics of core and are subject to errors of knowledge of the exact condition of the nuclear reactor core, especially the amount and distribution of fuel burnup, and are also subject to judgments made by the designers of the calculational programs.
The power distribution determination within large heavy water reactors, in contrast to the above described practices for light water reactors, have been largely based on measurements of gamma ray intensity rather than on neutron flux.
Recently, on a trial basis, the power distribution within a light water reactor was determined based on gamma ray intensity and the power distribution thus determined was found to be superior to a determination that was based on neutron flux measurements. The superior determination based on gamma ray intensity measurements results from the elimination of many of the above described errors in neutron flux measurement. However, the gamma ray detectors which were used for these measurements employed ionization chambers which are subject to failures from electrical breakdown and therefore have limited lifetime in the nuclear reactor environment.
The gamma ray detectors which have been employed in the large heavy water cores are called gamma thermometers. In the gamma thermometer, gamma flux is used to heat a strip of metal. One end of this metal is in direct contact with a heat sink and the heat produced by the absorption of gamma rays is transferred along the length to the heat sink. Typically, two thermocouples are used to measure the temperature gradient along the length between the uncooled end and the cooled end. Thus, the more intense the gamma ray flux, the higher the rate of heating of the strip of metal, and the higher the temperature gradient along the length of the strip. For a strip of uniform cross sectional area and cooled at one end as described above, the following equation applies: EQU .DELTA.T=qL.sup.2 /2K
where:
.DELTA.T=temperature difference PA0 L=distance between thermocouples PA0 K=thermal conductivity of the metal PA0 q=rate of heat generation within metal
The heavy water reactors in which these gamma thermometers have been utilized have alternate means for calibrating the gamma thermometers. Thus, in two different heavy water core types, it is possible to accurately measure the total power generated in nuclear fuel rods immediately surrounding a tube which contains several gamma thermometer detectors along its length. This provides a basis for calibrating the gamma thermometer signals in terms of surrounding reactor powder, but the calibration is incomplete because of large power intensity variations along the length of the nuclear fuel rod, and the calibration applies to the total signal output of the series of gamma thermometers along the length of the fuel rod rather than an individual gamma thermometer. Therefore, additional measurements are necessary in order to calibrate each gamma thermometer in terms of local power generation in the nuclear reactor fuel. This additional measurement includes determining the neutron flux distribution along the length of the nuclear reactor core. This neutron flux distribution is measured by momentarily inserting a long metallic wire alongside the tube of gamma thermometers while the nuclear reactor core is operating at power. The metallic wire becomes radioactive from neutron absorption, and the intensity of radioactivity induced varies along the length as the neutron flux varies along the length. Following a brief irradiation by neutrons within the nuclear reactor core, the metallic wire is withdrawn from the nuclear reactor core and the radioactivity per unit length is measured along the length utilizing appropriate measuring equipment. With this measurement and a knowledge of the construction of the reactor core and application of principles of nuclear reactor core neutron physics, it then is possible to determine the distribution of power generation along the length of the surrounding nuclear fuel rods described above. With this knowledge of power distribution along the length of the fuel rods, and also the total power along the length of the fuel rods as described above, it is then possible to calculate the local power of the surrounding fuel rods at all locations along the length of the surrounding fuel rods. Thus, it is possible to determine the calibration of the signal from each gamma thermometer in relation to the local power intensity of the fuel immediately surrounding each gamma detector. This calibration procedure is cumbersome and yields only a single point calibration that usually is attained when the nuclear reactor is at a relatively high steady power level. The calibration is subject to errors including those which result from calculating the local power generation based on neutron flux measurement.
The present invention is an improved method of calibrating gamma thermometers and is primarily intended for application in determining the power distribution in large light water moderated nuclear reactor cores that are employed as the energy source in nuclear electric power plants. The present invention provides greater accuracy in the determination of the power distribution in a nuclear reactor core, and thus makes it possible to operate the nuclear reactor core more effectively in terms of power distribution within the nuclear core and also the ability to operate the nuclear core at peak rated powers. The present invention can be employed in any type of nuclear reactor core, including the heavy water moderated reactors, liquid metal cooled reactors, gas-cooled reactors and other specialized reactor types.
The method of electric calibration of gamma thermometers yields much more accurate calibrations of the gamma thermometers than can be achieved by other means. This higher degree of accuracy of the gamma thermometer calibration then means that the power distribution within the nuclear reactor core may be more accurately determined than with present forms of power distribution monitoring for nuclear reactor cores, especially light water moderated nuclear power reactor cores.